The present invention generally relates to ac power control, and particularly relates to power regulation for ac power inverters.
Increasing use of localized power sources in residential and industrial settings coincides with the increasing desire to exploit potentially cheaper, cleaner, or more reliable alternative energy sources. Historically, individuals and businesses relied on central generation stations with power transmission and distribution networks, i.e., the traditional “grid,” almost exclusively but the developing trend is toward a number of smaller generating units, distributed geographically close to the loads. Local power generation using power sources such as photovoltaic systems, fuel cells, etc., also is becoming a viable option for businesses and homeowners. These local power generating systems can be configured as stand-alone systems that operate as the exclusive source of power for their local loads or they can be interconnected with the grid.
More commonly, these local power generating systems are configured as grid-connected systems that provide an opportunity for co-generation wherein power can be taken from the grid or provided to the grid. A grid-connected system allows a local load to remain connected both to the local system and to the power grid. In a typical setup, a local load is connected in parallel to an ac power inverter and to a utility grid. The load can be powered solely by the inverter, by the grid, or can receive a portion of its power from both.
Grid-connected inverters pose certain hazards, with undetected islanding standing as a primary hazard. With islanding, a portion of the utility grid becomes disconnected from or otherwise isolated from the rest of the grid but remains energized at potentially hazardous levels because of its connection to one or more local power inverters. Unless the connected power inverters recognize the islanding condition, the isolated portion of the grid remains energized even if other parts of the grid are de-energized, creating a potentially hazardous condition for service personnel and others.